Metal oxides have a great potential in various applications due to their interesting physical properties, such as superconducting, semiconducting, ferroelectric, piezoelectric, pyroelectric, ferromagnetic, optical (electro-optic, non-linear optic, and electrochromatic), resistive switching, and catalytic behaviors. Nano-scaled oxide materials have attracted great interest in the last decade because they can exhibit different physical properties than their bulk counterparts. See U.S. Pat. No. 6,036,774 by Lieber, et al “Method of producing metal oxide nanorods” issued on Mar. 14, 2000; Huang et al., SCIENCE, Vol. 292, p. 1897 (2001); Aggarwal et al., SCIENCE, Vol. 287, p. 2235 (2000); Li et al., Applied Physics Letters, Vol. 82, p. 1613 (2003); Luo et al., Applied Physics Letters, Vol. 83, p. 440 (2003). The oxide nanostructures were prepared by several growth techniques: laser ablation, sputtering, chemical vapor deposition, sol-gel, and molecular-beam-epitaxy. One of the simplest methods is to prepare the nanostructures, such as nanorods, in a tube furnace by the ‘vapor-liquid-solid’ mechanism suggested by Lieber et al.
Huang et al. demonstrated room-temperature ultraviolet lasing in ZnO nanowire arrays. The nanostructures were used as an optical cavity for lasing. Aggarwal et al. suggested their spontaneously formed oxide “nano-tip” array as a possible candidate for field emission applications. Li et al. presented an approach to use individual In2O3 nanowire transistors as chemical sensors, where ultrahigh surface-to-volume ratios were expected to improve the sensitivity. Luo et al. fabricated ferroelectric nanoshell tubes using Si and alumina hole arrays as templates. The nanoshell tubes could be useful for nano-electromechanical system. These results show that nanostructures can be useful for their unique structural advantages.
Carbon nanostructures, such as nanotubes, nanofibers and nanocones, (collectively “CN”) and their peculiar characteristics, such as field emission and field effect transistor effects, have also evoked great attention. In recent years, growth techniques for CN were intensively investigated and relatively well established. See Ren et al., SCIENCE, Vol. 282, p. 1105 (1998); Bower et als., Applied Physics Letters, Vol. 77, p. 830 (2000); Merkulov et al., Applied Physics Letters, Vol. 79, p. 1178 (2001); Tsai et al., Applied Physics Letters, Vol. 81, p. 721 (2002); Teo et al., Nanotechnology, Vol. 14, p. 204 (2003).
High-quality single-walled carbon nanotubes are typically grown as randomly oriented, needle-like or spaghetti-like, tangled nanowires by laser ablation or arc techniques (a chemical purification process is usually needed for arc-generated carbon nanotubes to remove non-nanotube materials such as graphitic or amorphous phase, catalyst metals, etc). Chemical vapor deposition (CVD) methods such as used by Ren et al., Bower et al., and Teo et al. tend to produce multiwall nanotubes attached to a substrate, often with a semi-aligned or aligned, parallel growth perpendicular to the substrate. Also Merkulov et al., Tsai et al., and Teo et al. demonstrated that carbon nanofibers and nano-cones can be grown in optimum conditions, for example by varying gas ratio and voltage bias.
As described in the cited articles, catalytic decomposition of hydrocarbon-containing precursors such as ethylene, methane, or benzene produces CN when the reaction parameters such as temperature, time, precursor concentration, flow rate, are optimized. Catalyst layers such as thin films of Ni, Co, Fe, etc. are often patterned on the substrate to obtain uniformly spaced CN array. Furthermore, the patterning of catalysts makes it possible to tailor the geometry (diameter controlled by catalyst size, height controlled by deposition time) of CN the demands for various applications. The catalyst dots can be patterned by various techniques: self-assembly, unconventional lithography (for example, nano-sphere lithography), and e-beam lithography. Careful patterning and growth enables production of carbon nanotubes with remarkable uniformity in diameter and height (standard deviations ˜5%), as reported by Teo et al.
While oxide nanostructures can be fabricated using various available techniques, the most frequently desired structural configurations such as well-defined, vertically aligned and periodically spaced nano oxide wires are not easily obtainable. In addition, some of the unique structures, such as a hollow oxide nanotubes and hollow oxide nanocones, are not easily synthesized using conventional techniques. Accordingly there is a need for improved methods of making oxide nanostructures.